Known fixing methods of an electrophotographically formed toner image include heat fixing and pressure fixing. Heat fixing requires a rise time for a fixing means to be heated up to a prescribed temperature after turning on the power and afterward energy for maintaining the fixing means at that temperature. Pressure fixing, on the other hand, requires neither a rise time nor heating energy. However, fixing strength obtained by pressure fixing is not sufficient so that high pressure should be applied for providing a toner image resistant to rubbing.
Capsule toners comprising a core material containing a fixing component, and a hard outer shell, which completely covers the core material, have been developed for solving the above-mentioned fixing problems. The outer shell of a capsule toner is destroyed on pressure application, and the fixing component runs off and is fixed on an image-receiving medium, such as paper and an OHP sheet.
When colorant particles or magnetic particles are exposed on the surface of the capsule toner, if used as an electrically conductive toner, the fluidity of the toner is deteriorated, making it difficult to form a chain on the development roll. Since the development of an electrically conductive toner is greatly affected by an electric charge induced on the toner chain, the reduction of the chain size makes it difficult for the induction electric charge to reach near the photoreceptor, resulting in the deterioration of the developability and the image quality.
When a large amount of magnetic particles are exposed on the surface of part of the toner particles forming a chain, an induction electric charge can be easily taken into the interior of the toner through magnetic particles on the surface of the toner particles. This results in the toner to be electrically charged, causing a fog in a non-image area. It is thus important to prevent the magnetic particles from coming out to the surface of the shell.
Further, the capsule toner comprises colorant particles or magnetic particles incorporated therein for the purpose of coloring or providing magnetic properties necessary for electrophotography. However, if a large amount of colorant particles or magnetic particles are incorporated in the core material in the capsule, the core material has so high a viscosity that the fixable substance does not flow out, resulting in the deterioration of the image fixability.
In order to improve the image fixability, it is preferred to cause a major proportion of the colorant particles or magnetic particles to be unevenly distributed in the shell. However, when the colorant particles or magnetic particles are exposed on the surface of the shell, the corresponding portion shows a resistivity change. Therefore, the electrical resistance of the toner varies with the extent to which these particles are exposed on the surface of the shell, giving a wide variation in the electrical resistance of the toner from lot to lot. Further, where the colorant particles or magnetic particles are hydrophilic, the electrical resistance of the toner greatly depends on the environmental conditions. Accordingly, from these standpoints also, it is important to prevent the colorant particles or magnetic particles from being exposed on the surface of the shell surface.
If the fixing component oozes out of the capsule during the preparation or storage of the toner, the toner particles will undergo agglomeration or remarkable reduction in fixability. Therefore, the outer shell needs to inhibit the penetration of the fixing component. However, the conventional capsule toner is not satisfactory.
For example, JP-A-57-179860 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a capsule toner whose outer shell comprises polyurethane or polyurea prepared by the reaction of an isocyanate, water, and an amine or glycol component, and JP-A-60-222866 discloses a capsule toner whose outer shell comprises polyurethane or polyurea prepared by using one bifunctional isocyanate and at least one tri- or polyfunctional isocyanate.
However, in cases where the amount of magnetic particles to be incorporated is increased or where the shell thickness is decreased for capsule size reduction, the colorant particles or magnetic particles sought to have an increased concentration in the shell and be exposed on the shell surface. This causes an image deterioration or fogging, and also gives a wide variation of the electrical resistance of the toner from lot to lot. The electrical resistance of the toner greatly depends on the environmental conditions. Further, the colorant particles or magnetic particles pierce the shell, through which the fixing component, particularly a low-molecular-weight component would run out.